The scale of a map is 1cm: 12 km. find the actual distance for 12cm

This is simply a question about ratios. This ratio means that for every 1 cm on the map, there are 12 km in actuality. You can set this problem up in fractions:

1 cm           12 cm
   =   
12 km             x

To solve for x, we cross-multiply, for the moment ignoring units. Remember, your answer will be in km.

1*x=12*12

x=144

If x=144, then for 12 cm, there are 144 km.